Fix parallel agent spawning

Problem

Trying to spawn multiple agents in parallel for tasks like t-441, t-445, t-280.2.4 failed due to:

1. agentd: Requires full YAML frontmatter with toolchain and workspace for .md files - won't accept simple markdown prompts with just provider/model 2. agent CLI via tmux: Commands sent to tmux windows don't execute properly - shell waits at prompt 3. agent CLI via nohup: Background processes don't start - unclear why

Desired Behavior

# Should work with simple .md prompts (no container config required)
agentd run task.md --provider claude-code -n task-123

# Or simple text prompts
agentd run "fix the bug" --provider claude-code -n task-123

Fix Options

1. Make agentd detect non-container prompts and run agent locally (not in docker) 2. Add --local flag to skip containerization 3. Create a simpler agent-parallel script for non-containerized parallel execution

Acceptance Criteria

• Can spawn 3+ agents in parallel from CLI
• Each agent runs independently with its own provider/cost limits
• Can monitor all agents via agentd top or similar